Is this a good SCR test?

Torben · Feb 24, 2010

Hi all,

I have an SCR I need to test. I don't have a dedicated device for testing SCRs, although I do have a relatively cheap multimeter with a BJT socket on it it, if that helps. (This is part of the repair project discussed in this thread).

I've tried this test: Testing SCR - How To Test And Check Silicon Controlled Rectifier , and I wonder how trustworthy it is or whether it's at least a reasonable guide. I could always build a test circuit from the datasheet and measure that.

The important bit from the above page is:

If you don't have this tester for checking SCR, I'm showing you another easy way on how to test SCR fast. You need an analog meter set to X1 ohm. Place the red probe to the Cathode and black probe to the Anode pin. At this time the meter doesn't show any reading. Now gently move the black probe and touch the Gate pin (the black probe still touching the Anode pin) and you will notice the meter's pointer will kick as shown at the picture (low resistance).

Removing the black probe from the GATE pin (the black probe still touching the Anode pin) you would noticed that the resistance continues to be there (low resistance). This is due to the conduction of SCR as the meter battery is usually able to supply current more than the holding current. If at this stage you removed the black probe from the Anode pin and connect it back, the pointer will dropped back to infinity (high resistance). If the SCR could hold the resistance then the SCR is considered good. If it can't hold then the SCR is faulty.

By this test, my SCR is faulty. Not trusting the battery in the multimeter, I ran the test several times with the analog meter powered from the battery and several times powered from a bench power supply. All tests failed. The meter kicked when I touched anode to gate but failed to hold.

Thanks for any help,

Torben

mneary · Feb 25, 2010

In order for the SCR to hold when you release the gate stimulus, the meter must be supplying enough holding current. Also, the red and black meter leads didn't always follow this author's convention. I have some of each polarity.

Can you use another meter to measure the short circuit current, polarity, and the open-circuit voltage of your analog meter? What's the SCR holding current spec?

transistor495 · Feb 25, 2010

Have you gone through sullivan county?
No need to worry about all these terminologies actually. The first thing you need to identify is whether it is an SCR or not. Then carefully remove those item from the chassis and perform this simple but practical test. I've tested and I got latching successfully.

More info about this practical test is available on the following link(sullivan county):
Basic Triacs and SCR Operation/Projects
Look at those industrial odd looking SCR's.

Hero999 · Feb 25, 2010

I didn't know there was any other way to test an SCR, other than to use a couple of pushbutton switches, a resistor and a bulb/LED.

The same basic circuit can be used to test BJTs and MOSFETs except the bulb goes of when the switch is released.

Torben · Feb 25, 2010

mneary said:
In order for the SCR to hold when you release the gate stimulus, the meter must be supplying enough holding current. Also, the red and black meter leads didn't always follow this author's convention. I have some of each polarity.

Can you use another meter to measure the short circuit current, polarity, and the open-circuit voltage of your analog meter? What's the SCR holding current spec?

Thanks! I'll give that a shot when I get off work today.

Your question about measuring the meter almost makes me wish I'd slept on the problem--it's so obvious and yet didn't occur to me last night. I appreciate the hint.

Torben

Torben · Feb 25, 2010

transistor495 said:
Have you gone through sullivan county?
No need to worry about all these terminologies actually. The first thing you need to identify is whether it is an SCR or not.

If it's not an SCR then they printed the wrong information on the case (or else I found the wrong datasheets for it). It says 2N6504--which should be an SCR.

Then carefully remove those item from the chassis and perform this simple but practical test. I've tested and I got latching successfully.
View attachment 39487

More info about this practical test is available on the following link(sullivan county):
Basic Triacs and SCR Operation/Projects
Look at those industrial odd looking SCR's.

Thanks for the link. I had actually found the basic testing circuit but was wondering if there was a reasonably reliable way to test using only meters--it would seem that as always the measurements will only be as good as the meters so I need to measure that too.

I'll do these tests (including the basic test circuit) when I get off work today and post the results.

Cheers,

Torben

Torben · Feb 26, 2010

mneary said:
In order for the SCR to hold when you release the gate stimulus, the meter must be supplying enough holding current. Also, the red and black meter leads didn't always follow this author's convention. I have some of each polarity.

Can you use another meter to measure the short circuit current, polarity, and the open-circuit voltage of your analog meter? What's the SCR holding current spec?

Measured this just now:

Short circuit current: 0mA
Polarity: Reversed?
Open circuit voltage: -1.55V

Sadly I'm too tired right now to build the simple test circuit. I reversed the leads on the analog meter and repeated the SCR test and got the same results. That only covers polarity, of course.

Torben

mneary · Feb 26, 2010

Check the current on the ohm meter again, 0mA (undetectable current) would be unusual. Your analog ohm meter should be showing a low reading when this test is done, equal to the resistance of the DVM you are measuring with.

Torben · Feb 28, 2010

mneary said:
Check the current on the ohm meter again, 0mA (undetectable current) would be unusual. Your analog ohm meter should be showing a low reading when this test is done, equal to the resistance of the DVM you are measuring with.

Just realized the fuse on the DVM is shot. I'll replace when I can and check that again.

The simple circuit test indicates that the SCR is just dandy. Pretty neat devices--I've read about them but haven't played with them before.

I'll check the rest of the circuit and test tomorrow while I'm watching hockey.

Thanks,

Torben

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